This invention relates to cable-coating extruder heads having die and guider assemblies with replaceable aperture-defining elements for accommodating different sizes of cable and/or producing different coating thicknesses, as well as to devices and procedures for changing these elements in such heads. As used herein, the term "cable" refers both to single-strand wire and to multistrand cable.
In present-day commercial practice, to provide electrical conductor cable with an insulating or like coating, the cable to be coated is continuously advanced longitudinally through a central passageway in an extruder head and through a die aperture at the forward end thereof while heated molten or otherwise fluent polymeric coating material (e.g. polyethylene containing a filler, a cross-linking agent, and/or other additives) is forced into and through the head by an extruder screw so as to come into coating contact with the advancing cable. Thus, the cable emerging through the die aperture carries a layer of the coating material, having a thickness determined by the clearance between the die and cable. A guider within the central passageway centers the cable in the die aperture as required for uniformity of coating thickness. Beyond the head, the coated cable typically passes through a steam chamber or other zone providing appropriate conditions for curing the applied coating. Detailed reference will be made herein to such production of insulating coatings on conductor cable, as constituting one important and illustrative specific use of the invention.
Specifically, the present invention relates to improvements in the broadly known type of extruder head, for coating a longitudinally advancing cable with a layer of fluent coating material, which comprises an extruder head body having a rearward end, a forward end, a central passageway extending between the ends and arranged for substantially axial advance of the cable therethrough, and a lateral passageway for delivering the coating material under pressure to the central passageway from a locality (e.g. a screw extruder barrel) outside the head body; a die extending across the central passageway at the forward end of the head body and defining a die aperture disposed and dimensioned for advance of the cable with clearance therethrough from the central passageway; and guider means mounted within the central passageway rearwardly of the die for centering the cable in the die aperture, the guider means having an annular lip for closely surrounding the cable adjacent the die and cooperating with the die to define an annular gap around the cable, the guider means further cooperating with the head body to define a passage for conducting the coating material from the lateral passageway through the gap into contact with the cable to apply thereto a coating material layer having a thickness determined by the clearance between the die and the cable. It will be understood that terms such as "front" and "forwardly," and "rear" and "rearwardly," are used herein with reference to the direction of cable advance; i.e. the rearward end of the extruder head body is the end at which the uncoated cable enters the central passageway, and the forward end is the end at which the coated cable emerges therefrom.
Advantageously, the same extruder apparatus is used at different times to coat cable of various different gauges and to produce coatings of various different thicknesses. When it is desired to change the gauge of cable being coated and/or to change the thickness of the applied coating, it is necessary to change the guider and/or the die so as to provide an appropriately dimensioned die aperture and guider lip for the new cable gauge and/or coating thickness. That is to say, since the guider lip must conform very closely to the cable diameter for accurate centering of the cable in the die aperture, and since the die-cable clearance determines coating thickness, production of a coating of a given thickness on a cable of given gauge requires use of a specific die and guider combination different from that required for any other gauge and coating thickness. In conventional extruder heads of the type described above, such changes have heretofore necessitated partial disassembly of the head body, viz. removal of the forward end of the head body, with attendant difficulties and inconveniences.
Thus, it has been conventional to provide the head body with a removable forward end portion, bolted to the central portion of the head body, and carrying the aperture-defining die element in a die holder which is in turn mounted and centered in the forward end portion of the head body by plural adjusting screws. The lip-bearing guider element has been threadedly secured in a central passageway in such manner as to be removable only through the front end of the passageway, i.e. after the forward end portion of the head body is dismounted. Change of die and guider elements therefore involves the successive steps of removing the forward end portion of the head body (thereby opening the front end of the central passageway); extracting the die element; unscrewing and withdrawing the guider element forwardly; inserting a new guider element; placing a new die element in the die holder; remounting the forward end portion of the head body, with the new die; and, finally, manipulating the adjusting screws to center the new die aperture properly relative to the guider lip. While the front end of the central passageway is open during this changing operation, fluent coating material bleeds out of the head, because the coating material must continue to flow in order to prevent the material from setting up in the extruder apparatus; this bleedout is wasteful of coating material, and also requires more or less extensive cleaning up of the equipment and work area. The centering screw adjustment is difficult and time-consuming, and tends to result in production of scrap (unevenly coated cable) at the start of the new run until proper centering is achieved. Overall, the down time required for each change of die and guider elements has typically exceeded one and one-half hours, with resultant impairment of desired productivity and efficiency of operation.